Construction method of deep foundation pit fender post horizontal inner support conversion structure

The invention relates to a construction method of a deep foundation pit fender post horizontal inner support conversion structure. The construction method comprises the steps that (1) floor stand column foundation piles are constructed at the bottom of a deep foundation pit; (2) reinforcing steel bars of a first longitudinal floor slab girder, a first transverse floor slab girder and a first ring beam are bound to the top of a first floor slab stand column, and a steel pipe stand column supporting system is installed close to a fender post; (3) a first horizontal inner support in the deep foundation pit is dismantled, and then a first basement outer wall is constructed on the inner side of the fender post; (4) the steel pipe stand column supporting system is installed on the floor slab girder, and the first floor slab stand column extends upwards to form a second floor slab stand column; and (5) gradually converting is conducted from bottom to top in a layered manner until the conversion construction of the h